The invention is particularly applicable to large format document scanners, i.e. those for scanning documents of widths greater than approximately 11 inches (30 centimetres). Typically, a document scanner has an image detection system which obtains image data for a single image line, and a scanning mechanism for causing relative movement between the detection system and a document so as to cause the image line to be swept over the surface of the document. This captures data representative of a succession of scan lines for the document, which can be reconstructed to provide an image of the document.
The image detection system of a document scanner will include an image sensor unit having an array of detectors, for example charge coupled devices (CCDs) or CMOS detectors. In the course of manufacture of the devices, there is a risk of failure of the device which increases with the number of detectors in the array, so that sensor units with arrays of a large number of detectors are more expensive to manufacture than devices which have a smaller number of detectors in their arrays.
Consequently, it is known for large document scanners to use a number of image sensor units, each of which captures data for a respective portion of each scan line. In order to ensure that corresponding portions can be assembled to provide a single continuous scan line, it is known for the ends of neighbouring portions to overlap, so that each portion has a slight overlap with one or two other portions, dependent on whether the portion is from an end or the middle of the scan line.
In a known method of generating image data representative of the entire scan line from the data from a plurality of imaging devices, an initial and/or terminal portion of the image data from each device, corresponding to the slight overlap or overlaps, is discarded and the remainders of the image data are concatenated with one another.
A relatively compact and low cost image sensor unit that can be used as an imaging device is a contact image sensor (CIS). This type of device has a linear array of image detectors, such as charge coupled devices or CMOS devices, covering an area similar to the area of the scan line portion associated with the device. This correspondence between the area of the detectors and the area to be scanned by the device means that bulky and/or expensive optical scaling systems are not needed. However, the need for an overlap between neighbouring scan line portions requires that the contact image sensors are arranged in a physically overlapping relationship in the direction in which the documents are scanned. Because the detectors of the devices have operating circuitry that needs to be positioned adjacent to the detectors, there is a limit to the minimum distance between the adjacent scan line portions in the scanning direction.
This means that the scan line portions must be re-matched in the direction of scanning, either in the electronics of the scanner or in software. Any inconsistencies in the motion of the paper across the CIS units can result in an error in stitching the images from the CTS units together.